The present invention relates to an intercell handover method, a radiocommunication system with mobile terminals implementing the method and switching equipment intended to be used in such a system.
It relates to the field of digital radiocommunications with mobile terminals, and finds applications in particular in private radiocommunication systems for professional users.
Such systems are in general cellular systems, in the sense that they include a radio subsystem (forming part of the fixed network) having base stations distributed over the geographic area covered by the system, the area covered by a respective base station being called a cell. The available radio resources (radio frequencies) are distributed among the various base stations to provide communications with mobile stations. This means that the same radio resources can be reused in non-adjacent cells, without incurring problems of interference between calls, and therefore means that the spectral efficiency of the system can be increased.
When a mobile station on a call moves from a determined cell, referred to as “source cell”, to a neighboring cell, referred to as “destination cell”, the call must be handed over to a frequency allocated to said destination cell. This operation is well known in cellular radiocommunication systems and is commonly called intercell handover.
Intercell handover is commanded, either through the action of the mobile station, or through that of the fixed network, according to (a) a first indication on the quality of the radio link between the mobile station and the base station of the source cell, and (b) a second indication on the quality of the radio link between the mobile station and the base station of the destination cell. These indications are measured by the mobile station. If the intercell handover takes place through the action of the fixed network, these indications are furthermore transmitted to the fixed network via a special-purpose signaling channel. The fixed network commands the intercell handover when the second indication becomes better than the first.
To prevent the intercell handover taking place too early, which would run the risk of leading to its failure, with the consequence of the possible loss of the call, the provision of a hysteresis is also known, for the comparison between the first indication and the second indication.
Measurements, performed by the mobile station to evaluate the first and second indications, take place during polling time slots, during which the mobile station is neither receiving nor transmitting any signal related to the call in progress or to signaling. These measurements are based on the polling of the beacon frequency of the destination cell. The first and second indications are evaluated, by the fixed network, by taking a statistical mean over several such measurements.
In practice, since it is impossible to know in principle which neighboring cell of the source cell the mobile station is moving to, the mobile station polls the beacon frequencies of a plurality of neighboring cells, the references of which are transmitted to it by the fixed network via a special-purpose signaling channel. In fact, there is therefore a plurality of second indications on the quality of the radio link between the mobile station and the base station of each respective neighboring cell.
In certain systems, the measurements to evaluate the first and second indications (one second indication per neighboring cell considered) are performed with a relatively long period between two consecutive measurements due to the low recurrence of polling time slots. This is for example the case for FDMA systems, that is frequency division multiple access systems, or for low order TDMA systems, that is time division multiple access systems, for example of order 2 (TDMA-2). It follows that the decision to command an intercell handover is taken by the fixed network based on measurements for which the statistical value is relatively low. The result of this can be intercell handover decisions that are too early or too late. In the first case, the call risks being lost. In the second case, the mobile station risks becoming a source of interference for the calls set up on the same frequency in other cells of the network, and this is all the more so since a control mechanism causes the transmit power to increase when the mobile station moves away from the base station with which it is communicating.
The invention aims to overcome the abovementioned drawbacks of digital radiocommunication systems with mobile terminals.
To this end, the invention proposes the implementation of a technique called intercell soft handover. This technique has become known through its application in code division multiple access (CDMA) systems. The principle of this technique relies on an absence of breakage of the call during the transition from one cell to another. For CDMA systems, in which the same frequency can be used in two neighboring cells, one implementation of the principle consists in allocating in two neighboring cells the same code for a given call. This enables the mobile station to be received, for the entire cell change duration, both by the source cell and by the destination cell, and thus to pass softly from one to the other.
Examples of application of the intercell soft handover technique are given in references U.S. Pat. No. 6,073,021 and EP-A-0 797 367. The techniques written up in these documents apply however only to CDMA systems.
There are many advantages of this technique. First, it enables minimization of failures during intercell handovers since at no moment is there a sudden change in radio link and since, due to the availability during the critical phase of two radio links whose quality levels add together, the risks of a resulting poor quality radio link diminish considerably. Secondly, due to the fact that the intercell handover is not irreversible during its critical phase, the decision can be taken earlier, while the quality of the link with the source cell is still relatively satisfactory, thus avoiding the situation in which a mobile station moving away from a cell transmits at too high a power level due to its separation from the base station.
Adapting the principle to systems other than CDMA systems, and in particular to the GSM system, was proposed for example in document GB-A 2 338 376 (WO 99/65264). Since the normal intercell handover mechanism proposed by GSM (TDMA system of order 8) is an operation that is well suited to the normal cellular mode, this document proposes a solution for picocells provided in the densest urban areas. A set of picocells in fact forms a single cell in the sense of the cellular system considered (GSM in this case), this cell having a single BCCH control channel, transmitting in simulcast mode at a particular frequency. Since, according to the GSM standard, this frequency must be transmitted continuously, a certain number of traffic channels (in general seven since the BCCH channel occupies only one time slot per frame on a carrier in a TDMA-8 system such as GSM) also turn out to be transmitting in simulcast mode. From that moment, a mobile station communicating over a traffic channel dedicated to one of the picocells, and whose link quality falls below a certain threshold, undergoes a handover internal to the cell (called intracell handover), bringing it to one of the traffic channels in simulcast mode. The indications on the quality of links in the uplink direction on this channel, which is transmitted and received by the various base stations of the picocells, are used to quickly determine the picocell to which the handover must take place. A second handover internal to the cell is then performed to this picocell, completing the change of picocell.
This mechanism, based on a control channel whose coverage is much greater than the traffic channels to which it gives access, applies only to the case of systems operating in very dense environments and which, since they require a high frequency reuse rate, have a network of picocells superimposed on the network of cells.
However, these picocells are not cells in the usual sense of the term, since they do not have a specific control channel. This mechanism applies only to high ranking TDMA systems since a certain number of traffic channels at the same frequency as the single control channel of the configuration is required, which is impossible with an FDMA or low ranking (for example 2) TDMA system.
In addition, it assumes the existence of traffic channels in simulcast mode having the same coverage as the control channel, which naturally turns out to be the case in the GSM system but can turn out to be more difficult, even impossible, to achieve in other systems.
Document U.S. Pat. No. 5,278,991 proposes a similar technique in which an umbrella macrocell covers a certain number of microcells. This technique differs from the one described in the abovementioned document mainly by the fact that the microcells are authentic cells and each has a respective broadcast control channel (BCCH channel). Nevertheless, it also applies only to multilayer systems, comprising a contiguous macrocell layer covering a layer of microcells that are not necessarily contiguous.
Document EP-A-0 876 005 discloses an intercell handover technique according to which there exist frequencies common to all the cells, at one of which frequencies the mobile station transmits during the intercell handover procedure. However, the technique described requires that the mobile station changes frequency to transmit on this common frequency, prior to executing the intercell handover. In addition, this technique requires that common frequencies are assigned exclusively to the various base stations for executing the intercell handover procedure. In addition, this technique means that the base stations do not transmit at the frequency paired with the common frequency of reception by the base stations. Finally, it assumes that the list of cells involved in the intercell handover procedure is defined in advance.